1. Field of the Invention
The present invention relates to a method and system for wireless communications suitable for P-MP wireless communications.
2. Description of the Related Art
High-speed wireless data communications standards (IEEE802.16) have been specified in Non-Patent Document 1. In the physical layer of IEEE802.16, Single Carrier, OFDM (Orthogonal Frequency Division Multiplexing), and OFDMA (Orthogonal Frequency Division Multiplexing Access) have been supported as modulation systems. Hereinafter, description will be given of a wireless data communications method compliant with IEEE802.16 with reference to a case where OFDM is adopted as the modulation system, for example.
In IEEE802.16, P-MP (point-to-multipoint) architecture has been prescribed as one of the communication modes, and in the P-MP, a base station (BS) schedules transmission/reception opportunities of all mobile stations (MSs), whereby making it possible to carry out efficient communications to which QoS (Quality of Service) has been ensured.
FIG. 16 shows an example of an OFDM frame configuration in P-MP, in which scheduling information is stored in a MAP message included in a broadcast message field within a first DL (downlink) burst following a preamble and an FCH (Frame Control Header).
In the scheduling information, downlink slot information and uplink slot information to be assigned to each MS are stored. As a result of receiving these types of information, the MS can find out the time at which data addressed to itself arrives and the time at which it may transmit data. Hereinafter, the MAP message included in the broadcast message field within the first DL burst (#1) following the FCH is expressed as a MAP message.
Each BS is provided with a 48-bit Base Station ID (BSID), and the MS identifies each BS based on the BSID. High-order 24 bits of the BSID are unique to each service provider, and within the identical service provider, each BS is identified based on low-order 24 bits.
FIG. 17 shows a frame structure of a MAC header prescribed in IEEE802.16, and in each of the connections, a unique connection ID is provided for each connection. In the MAC header according to IEEE802.16, a space for the connection ID is prepared, and the BS and MS identify a packet based on the connection ID.
Each MS periodically carries out a ranging process when joining and after joining a network so as to adjust transmission/reception timing with the BS and transmission power. When joining a network, each MS carries out ranging in an initial ranging period assigned by the BS. This initial ranging period is of a contention base, and the MS transmits a RNG-REQ (ranging request) message to the BS in this period. The BS that has received the RNG-REQ registers information such as transmission power, frequency, and transmission timing in a RNG-RSP (ranging response) message with a ranging continue notice (Ranging Status in RNG-RSP=continue) so as to respond to the MS if adjustment of the MS is necessary.
The MS that has received the RNG-RSP with the ranging continue notice adjusts transmission power, frequency, and transmission timing and then transmits RNG-REQ again to the BS in an initial ranging period assigned to each MS by the BS. If the transmission power and the like of the MS satisfies a preset value, the BS responds to the MS with RNG-RSP with a ranging success notice (Ranging Status in RNG-RSP=success).
Such ranging ends with the RNG-RSP with the ranging success notice being received by the MS. Ranging that is carried out after the MS has joined a network is carried out in an initial ranging period the same as that when it joins a network or in an initial ranging period assigned for the MS.
In a case of OFDMA, the above-described ranging steps uniquely identify an MS by a CDMA base using a code prescribed in Non-Patent Document 2 prescribed within a pre-assigned “ranging subchannel” range (FIG. 18).
The BS transmits data to an SS by use of a burst profile described in “Downlink Operational Burst Profile” of the RNG-RSP. In addition, the MS transmits data to the BS by use of a burst profile described in UIUC of a UL-MAP. The burst profile described in UIUC of the UL-MAP is specified by the BS. The burst profile means a parameter set including a modulation system, an FEC code type, and the like.
[Non-Patent Document 1] IEEE Standard for Local and metropolitan area networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems, IEEE Std 802.16-2004, Oct., 2004.
[Non-Patent Document 2] IEEE Standard for Local and metropolitan area networks—Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems, Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigenduml, IEEE Std802.16e-2005, February 2006.
In wireless data communications, owing to an influence of radio attenuation caused by shielding by an obstacle located between transmission and reception antennas, there is a blind zone where an MS cannot communicate with a BS despite being in a service area. For high-speed wireless data communications, it is necessary to use a high frequency, however, with a high frequency, the wavelength is short and rectilinear propagation characteristics of radio waves increase, therefore, a further increase in the blind zone is expected. As a method for eliminating such a blind zone, it can be considered to install a relay station (RS) between the BS and MS.
However, when the RS is added to a P-MP wireless data communications system, inequality occurs when MSs that directly communicate with the BS are compared with MSs that indirectly communicate with the BS via an RS in terms of communications opportunities of a ranging request and communications opportunities of a bandwidth request. Namely, for the MS that indirectly communicates with the BS, since it becomes necessary to have a communications opportunity at which the MS itself makes a request to the RS and a communications opportunity at which the RS makes a request further to the BS, the MS is disadvantageous in comparison with the MS that directly communicates with the BS.
Furthermore, if a relay station is added to the P-MP, it is necessary for the BS to hold information on SSs under the base station and to provide connection IDs for the SSs. Here, if a passive relay that simply transmits received data after shaping and power amplification is used as the relay station, since the BS cannot know the transmission/reception environment between the RS and SS, a suitable modulation system cannot be selected.